The present invention relates to a method for producing a polygonal mirror.
In an apparatus employing a laser beam such as, for example, a laser printer, a polygonal mirror is used as a means to use a laser beam emitted from a laser beam emitting source for scanning through a deflection.
The polygonal mirror is a polygonal plate-shaped object, and a reflection surface is provided on each side of the polygonal mirror. When the polygonal mirror is rotated so that each reflection surface moving on a circular motion basis may receive a laser beam, the laser beam is deflected for scanning.
There is recently used a method wherein a reflection surface of a polygonal mirror is formed by mirror-machining a metallic material composed of an aluminum alloy or the like with a diamond tool.
FIG. 8 is a schematic diagram showing a shape of a diamond tip on a diamond tool.
A diamond tip shown in FIG. 8 (a) is one wherein semicylindrical rake face 1 is formed, and circular-arc-shaped end cutting edge 3 is formed by both rake face 1 and front flank 2 (hereinafter referred to also as "one with R-shaped rake face"), while a diamond shown in FIG. 8 (b) is one wherein circular-arc-shaped end cutting edge 3' is formed by both flat rake face 1' and curved front flank 2' (hereinafter referred to also as "one with R-shaped nose"). Incidentally, in FIG. 8, the numeral 4 represents a side flank and the numeral 5 represents major cutting edge.
When obtaining a reflection surface of a polygonal mirror by the use of the diamond tools mentioned above, a metallic material is machined by major cutting edge 5 first. Then, the machined surface is subjected to burnishing by means of end cutting edge 3 (3') to remove scratches or the like for machining a mirror-like surface.
However, with regard to a cutting tool with an R-shaped rake face and a cutting tool with an R-shaped nose, it is difficult to machine accurately their cutting edges. Despite an endeavor to make cutting tools to be in the same shape in production, errors in shape can not be avoided, resulting in occurrence of deviation in shape. Namely, it is actually difficult to grind a monocrystal diamond which is anisotropic in crystal accurately to a shape of a semicylindrical surface or a curved surface, and it is not possible to form a shape of a circular arc of major cutting edge 3 (3') highly accurately, resulting in defects such as fine chipping or a waviness on a cutting edge formed which tend to occur. When machining a reflection surface of a polygonal mirror using a cutting tool having the defects mentioned above, fine wariness and unevenness are caused on the machined surface, bringing occurrence of scatted light and a fall of reflectance which greatly deteriorates efficiency of the polygonal mirror, though mirror surface seems to have been obtained superficially. A method for producing a polygonal mirror by the use of cutting tools having an R-shaped rake face and an R-shaped nose can not be free from occurrence of defective products and from a fall of production efficiency as stated above. The method, therefore, is far from an appropriate one for producing a polygonal mirror.
On the other hand, inventors of the invention studied how to mirror-finish a reflection surface of a polygonal mirror using a diamond tool wherein both rake face 6 and front flank 7 of a diamond tip are flat and both major cutting edge 8 and end cutting edge 9 are straight in shape (hereinafter referred to also as "flat edge form") as shown in FIG. 9.
Mirror-finishing by means of a cutting tool of a type of the flat edge form is carried out as follows.
First, the diamond tool is brought into contact with a metal material on which a reflection surface is to be formed. In this case, the diamond tool is caused to contact so that major cutting edge may be inclined by a predetermined angle (entering angle) from a feed direction for the diamond tool and end cutting edge may be in substantially parallel with the feed direction for the diamond tool.
Then, as shown in FIG. 10, the diamond tool is fed with a constant pitch (feed direction is shown with "arrow v" in FIG. 10). Thereby, metal material W is cut by major cutting edge 8. On the surface of the metal material W cut by major cutting edge 8, there occurs scratches p or the like, and the surface where major cutting edge 8 has just passed is compressed and lifted (a level on the surface where major cutting edge 8 has just passed is shown with one-dot chain lines). The surface thus compressed is restored by the elasticity of the metal material W (a level on the surface in restoration is shown with solid lines in the figure), and scratches p and the like existing on the surface are burnished or removed by the succeeding end cutting edge 9. Owing to such burnishing by means of end cutting edge 9, a mirror surface can be formed.
Since major cutting edge and end cutting edge of a diamond tool with flat cutting edges as those mentioned above are straight in form, it is possible to consider crystal orientation when grinding the cutting edges. In a diamond tool with flat cutting edges thus obtained, an error in its shape is small and its cutting edges are free from defects of cutting edge chipping and waviness to be accurate in shape. Therefore, it is possible to control, to a certain extent, occurrence of defective products caused by deviations in accuracy of a cutting tool and a fall of production efficiency.
However, when obtaining a reflection surface of a polygonal mirror by the use of a cutting tool with flat cutting edges, a thickness of a chip produced through cutting by means of a major cutting edge is greatly different from that of a chip produced through burnishing by means of end cutting edge. Therefore, chips produced in the course of burnishing are broken into pieces to become cutting powder which enters between a metallic material and the cutting tool, causing sometimes scratches. In this case, it is not possible to form a mirror surface stably.
The invention has been achieved in view of the circumstances mentioned above, and its object is to provide a method for producing stably polygonal mirrors having no scattering and having high reflectance wherein efficiencies of cutting and burnishing can be kept constant.